[Prev][Next][Index][Thread]

Re: Primary Coil Design

> I,ve been working with induction heaters in the 200 kHz to 600 kHz
> frequency range for about 5 years now.  One common material that is
> easy to heat inductively is graphite. It has  a resistivity about 800
> times higher than copper.  This means it gets hot a lot faster than
> copper does in the same RF field.
> 
> A slug of graphite placed at a spot where you suspect you have an RF
> heating problem would get hot long before any other object would.  Stick
> a cheap alcohol thermometer (the kind with the red fuild)in the graphite
> slug and you have a primitve detector.  Don't use a mercury thermometer
> since mercury is an electrical conductor.  A half inch cube of graphite
> would be about the right size. Drill a hole and epoxy the thermometer in
> place.  If you leave the scale on make sure it is plastic and remove any
> metal bits used in its construction.
> 
> Note that graphite powder won't couple to the RF fields frequencies normally
> used by TC buildes.  I use graphite powder as a thermal insulator for a
> graphite crucuble that I heat inductively up to 1800 degrees centigrade
> (3242 degrees F).  The graphite powder just doesn't see the 400 kHz RF.
> 
> I work at power levels ranging from a few hundred watts up to 25 kilowatts.
> Below about 1000 watts you have to get within a couple of inches of the
> coils before things start to get hot but then my objective is usually to melt
> anything I'm heating.  Also, all my conductors are either 1/4 or 3/8 inch
> water cooled copper tubing, so I never notice them getting hot unless I forget
> to turn on the cooling water.  The smoke usually gets my attention.
> 
> Hope this helps.
> 
> Harry Adams


Harry,

Thanks for such a great tip. I'm sure everybody thumbing through their 
junk parts catalogs looking for graphite chunks right now! I know I will 
be.

Any ideas on the the loss of Q and the gaining of stray inductance of 
lead distances between 12" and 20"? I've got 1500+ straight strand, 
0000ga. copper cable for the connections between the capacitors and the 
primary coil. Please check out "www.ddlabs-dot-com/tesla.html" to see photos 
and renderings of this coil project.

...Jeff